Kinetochores and microtubules wed without a ring

Abstract

Proper chromosome segregation in mitosis requires tethering of spindle microtubules to the kinetochore. Using electron tomography of mammalian cells, McIntosh et al. (2008) now report the presence of fibrils that connect the inner kinetochore to the curved protofilaments at microtubule ends, suggesting a new model for force generation in chromosome movement.

Figure 1. Kinetochore-Microtubule Attachment

(A) Geometric configuration of conserved kinetochore components in budding yeast. The kinetochore provides the physical linkage between the microtubule plus end (green) and the centromeric DNA (wrapped around the centromere nucleosome, Cse4). The drawing reflects the number of individual complexes based on quantitative fluorescence microscopy, the structure of complexes from sedimentation velocity or electron microscopy, and the assumption that there is three-dimensional symmetry around the microtubule lattice. (B) The microtubule (green, right) is a 25 nm diameter filament. The ring (red) depicts the notion that an element in the kinetochore encircles the growing or shortening plus end of the microtubule. The centromere-specific histone (orange circle) is at the apex of a loop of intramolecularly paired pericentric chromatin. In contrast, the findings of McIntosh et al. (2008) suggest that 2–4 nm fibrils bind the inner surface of the curved protofilaments. These fibrils may be proteinaceous (black coiled-coil α-helical protein) or DNA (stretched pericentric chromatin).